Conventionally, a card reader has been widely utilized which is structured to read data recorded on a card and/or record data to a card. In an industry such as a financial institution where a card reader is utilized, illegal acquisition of magnetic data of a card by a criminal who attaches a magnetic head to a card insertion part of a card reader, in other words, so-called skimming has conventionally become a large issue. In order to cope with the issue, a card reader has been proposed in which a disturbing magnetic field is generated for preventing reading of magnetic data by a magnetic head for skimming (hereinafter, referred to as a “skimming magnetic head”) (see, for example, Patent Literature 1).
Further, conventionally, a differential type skimming magnetic head in which a read head and a dummy head having substantially same electrical characteristics are accommodated in one case body has been known as a skimming magnetic head. The read head is a head configured to abut with a magnetic stripe of a card and to read magnetic data. The dummy head is adjacently disposed to the read head in a width direction of a card and is disposed at a position where the dummy head is not abutted with a magnetic stripe. When magnetic data of a card are read by the differential type skimming magnetic head, an output signal of the read head becomes a signal superposing a signal corresponding to magnetic data to a signal corresponding to an external magnetic field, and an output signal of the dummy head becomes a signal corresponding to the external magnetic field. Therefore, in a case that magnetic data are read by a differential type skimming magnetic head, even when a disturbing magnetic field is generated like the card reader described in Patent Literature 1, a read signal of magnetic data which is not affected by the disturbing magnetic field can be obtained by obtaining a difference between an output signal of the read head and an output signal of the dummy head. Accordingly, skimming is not prevented.
Therefore, a card reader for preventing skimming by a differential type skimming magnetic head has been proposed (see, for example, Patent Literature 2). The card reader described in Patent Literature 2 includes a disturbing magnetic field generation part having two radiation sources configured to radiate a disturbing magnetic field. The two radiation sources are disposed so as to interpose a conveying passage for a card in an upper and lower direction. A distance between one radiation source and the read head and a distance between the one radiation source and the dummy head are different from each other, and a distance between the other radiation source and the read head and a distance between the other radiation source and the dummy head are different from each other.
Therefore, in the card reader described in Patent Literature 2, a difference is generated between an amplitude (intensity) of an output signal of the read head and an amplitude of an output signal of the dummy head caused by the disturbing magnetic field from one radiation source, and a difference is generated between an amplitude of an output signal of the read head and an amplitude of the output signal of the dummy head caused by the disturbing magnetic field from the other radiation source. Therefore, according to this card reader, even when a difference between an output signal of the read head and an output signal of the dummy head is obtained, a read signal of magnetic data which is not affected by the disturbing magnetic field is unable to be obtained and, as a result, skimming by a differential type skimming magnetic head is prevented.